Three-dimensional (3-D) imaging has recognized value in a number of applications as diverse as medical imaging, visualization technologies, and navigational guidance systems. Advantaged over 2-D display methods and devices, 3-D imaging techniques help to improve how well image data can be represented to a viewer and provide useful characteristics that allow a measure of depth perception that can be particularly valuable in such applications.
A number of 3-D imaging methods are available for more closely representing objects as they appear in space. Stereoscopic imaging apparatus, for example, operate by forming separate images for the left and right eyes of a viewer. Typically, some type of device, such as polarized glasses or other device, is needed in order to separate the two images and allow a measure of stereoscopic viewing. Auto-stereoscopic viewing apparatus can form a 3-D image without the need for a separation device, using virtual imaging methods for forming left and right eye pupils for the viewer at appropriate points in space. However, such devices must compensate for viewer movement out of the pupil space in order to successfully provide a suitable stereoscopic image.
While stereoscopic and auto-stereoscopic imaging apparatus provide the appearance of 3-D, however, the imaging methods that are used provide only a limited number of psychological depth cues. True depth perception is based on a complex interaction of the visual system and related brain processes that we use to recognize and locate positions in space. Stereoscopic and auto-stereoscopic systems simulate convergence which is an aspect of depth position, but fail to provide the visual cues for focal accommodation. Parallax, for example, is observable only over a limited viewing zone. The perspective of the stereoscopic pair is correct for only a small distance. Overall, stereoscopic depth cues can tend to conflict with physical cues, leading to visual confusion and fatigue, as well as to misjudgment of distance, velocity, and shape. This conflict is known in the art as the “convergence-accommodation discrepancy”. For reasons such as these, stereoscopic techniques can be inappropriate for navigational guidance applications, such as for use in heads-up displays.
Unlike stereoscopic methods, volumetric display methods actually form a true volume image that provides realistic physical depth cues, such as focal accommodation, parallax, convergence, and biocular disparity. A volumetric display operates by forming an image whose light rays, from the position of the viewer, are substantially indistinguishable from light rays that would appear to the viewer for an actual object.
One useful application of volumetric imaging systems is for use in navigational guidance. For example, International Publication No. WO 2005/121707 entitled “En-route Navigation Display Method and Apparatus Using Head-up Display” by Grabowski et al. describes a navigational display system that forms an image of an overhead cable or other element for guiding the driver of a motor vehicle to a destination, much in the manner of following a cable. The volumetric imaging apparatus that is used forms a true volume image as a virtual image using the windshield or other surface that is disposed in front of the driver. In the volumetric imaging optical apparatus, a light source is rapidly scanned along a screen or diffusive element for forming the image of a “virtual cable”. During each scan, the diffusive surface vibrates or uses some other method for rapidly changing the focus during a scan.
While the methods and apparatus taught in the '1707 Grabowski et al. application provide the benefits of volumetric imaging for improved navigational guidance, however, implementation of such a system comes at a cost. The optical system needed to support volumetric imaging is hampered by its size requirements and by the complexity of its aspherical optical components. Lenses used are characterized by large diameters, resulting in high cost, placement constraints, and difficulty of mounting.
In light of these and other considerations, desirable characteristics of an improved, commercially viable volumetric display for use in automotive and vehicular environments include the following:                a) high image quality;        b) correction of convergence errors induced by the windshield;        c) compactness of the imaging optics, without noticeable impact on dashboard, windshield, or instrument panel design; and        d) sizable eye-box and sufficiently large image area for comfortable viewing without eye-strain, with pupil sizing that accommodates the positioning of the driver, the vehicle dashboard, and the windshield.        
Thus, it can be seen that there is room for improvement in volumetric imaging for navigational guidance and other applications.